ATRX syndrome represents a combination of alpha-thalassemia, mental retardation, and multiple associated developmental abnormalities. The gene defective in ATRX has been localized to the X chromosome and recently cloned. Mutations in the same gene also cause several other forms of syndromal X-linked mental retardation. The ATRX gene encodes a gene product containing a SWI2/SNF2-type DNA-dependent ATPase domain. Thus, it has been hypothesized that ATRX could function in an ATP-dependent chromatin-remodeling complex and participate in regulation of gene expression. We established collaboration with Drs. Doug Higgs and Richard Gibbons, who had cloned the ATRX gene and have discovered important features of its function. Using our techniques, ATRX fractionated as a complex of 1.5 Mda by gel-filtration chromatography. The ATRX complex was immunoisolated using 4 antibodies against 4 different regions of the protein. Mass spectrometry and immunoblotting analysis revealed that the complex comprises ATRX and at least 7 other components (referred to as AXAPs for ATRX-associated proteins). Several AXAPs have been previously identified as transcription factors. For example, AXAP110 has been shown to act as a transcriptional corepressor. In a critical control, antibody against AXAP100 could immunoprecipitate ATRX. Immunofluorescence studies showed that these two proteins colocalize in vivo. Two other AXAPs have been previously identified as chromatin-dependent transcription factors. We have tested the ATRX complex for conventional mononucleosomes disruption activity. ATRX complex has an activity rather different from that of the SWI/SNF or NURD families of remodeling complexes. The ATRX complex seems to be able to disrupt 10 bp ladder of DNase I digestion pattern of a nucleosome in the absence of ATP, and the degree of disruption seems to be less in the presence of ATP. We are currently developing assays to study other biochemical activities of the ATRX complex.